Wireless data communication using inductive signals between an antenna coil of a transponder circuit arranged on a vehicle wheel and an excitation coil of an interrogation circuit mounted on the vehicle body is well known. An arrangement of the transponder circuit with its antenna coil on the wheel can be chosen such that data communication is, if possible, independent of the position of the transponder on the tyre in relation to the interrogation circuit. Data communicated between the transponder circuit and the interrogation circuit can be identification data and/or data concerning measurements carried out by one or several of the transponder circuit sensors.
As can be seen with reference to FIGS. 1a and 1b, transponder circuit 1 is for example mounted on an inner wall of tyre 6 of vehicle wheel 5 with a magnetic coupling coil 2. An interrogation circuit 3 fixed onto a fixed structure of the vehicle communicates data to the transponder circuit via a primary magnetic field 8 generated by an excitation coil 4.
The magnetic coupling coil 2 is mounted coaxially to the rotational axis that passes through the centre of the wheel rim 7 such that data communication with interrogation circuit 3 is, if possible, independent of the position of the transponder circuit on the wheel. This coupling coil can define a closed loop in order to act as an inductive interface between excitation coil 4 of interrogation circuit 3 and an antenna coil of the transponder circuit, which is not shown in FIGS. 1a and 1b. The magnetic coupling coil can also be directly connected to the transponder to act as the transponder circuit antenna coil.
In the example shown in FIGS. 2a and 2b, transponder 1 is directly connected to magnetic coupling coil 2 which thus acts as the transponder circuit antenna coil. Coupling coil 2 is preferably arranged at the periphery of the vehicle tyre, as described generally in DE Patent No. 199 24 830, which shows this embodiment. The primary magnetic field 8 originating from excitation coil 4 of interrogation circuit 3 induces a current in the antenna coil. This induced current also induces a secondary magnetic field 11 in an opposite direction to the primary magnetic field.
Since transponder 1 is directly connected to the magnetic coupling coil as the transponder circuit antenna coil, it can be integrated in the tyre structure in an initial phase of the tyre manufacturing steps. However, it may undergo mechanical stresses, which are linked to the temperature increase, and to the bending or compression of certain parts of the tyre when the vehicle is moving, which is a drawback, since in the event of a failure, it is relatively difficult to change the transponder circuit.
Transponder 1 can also be connected to the magnetic coupling coil after the tyre manufacturing steps by using a specific connector. However, with a connector of this type, over time and with the various stresses to which the tyre is subjected, a poor connection has been observed between the transponder and the antenna coil.
In the example shown in FIGS. 3a and 3b, transponder circuit 1 comprises its own antenna coil 10. A coupling coil 2 in the form of a closed loop acts as the inductive coupling interface between the transponder circuit antenna coil and an excitation coil 4 of interrogation circuit 3.
As can be seen in FIGS. 3a and 3b, when transponder circuit 1 is in proximity to excitation coil 4, induced magnetic field 11 of the coupling coil and primary field 8, which is opposite to induced magnetic field 11, passes through its antenna coil 10. Consequently, since the direction of the primary magnetic field is opposite to the direction of the induced magnetic field in closed loop coupling coil 2, a loss of data or energy can be observed at the moment when transponder circuit 1 is opposite excitation coil 4 of the interrogation circuit.
When transponder circuit 1 is far from excitation coil 4, only induced magnetic field 11 in coupling coil 2 transmits energy and data thereto. Despite the presence of the coupling coil, the transmission of energy or data is dependent upon the rotation of vehicle wheel 5 and thus upon the position of the transponder circuit on said wheel, which is a major drawback. This problem can also be observed when the transponder circuit communicates with the interrogation circuit.
In this regard, EP Patent No. 1 354 729 can be cited, which describes the arrangement of a transponder circuit with a magnetic coupling coil as described with reference to FIGS. 3a and 3b. However, the coupling coil and the tyre identification and monitoring transponder circuit are preferably mounted in a tyre tread. However, such an arrangement is also dependent upon the position of the transponder in relation to the excitation coil of an interrogation circuit, which is a drawback.
By way of alternative embodiment shown in FIGS. 4a and 4b, the magnetic coupling coil 2 can comprise a loop portion 2′ surrounding antenna coil 10 of transponder circuit 1. This has the effect of increasing the intensity of induced field 11 penetrating antenna coil 10 without modifying the dependence of the position of transponder 1 as indicated hereinbefore. When transponder circuit 1 is in proximity to excitation coil 4 of interrogation circuit 3, the resulting field penetrating the antenna coil is the sum of the primary magnetic field and the opposite sign induced field of the coupling coil, which is a drawback for data and electrical energy transmission.